Gapped signals are widely used in Optical Transport Network (OTN), broadcast video as well as many other application areas. One of the key requirements of an OTN is to insert plesiochronous payloads into an OTN wrapper. That is, because the data transmission rate and the rate of the payload source may not be exactly the same, they may drift with respect to one another. One approach to solve the issue to is to generate a reference clock with missing clock edges (or gaps) to keep the incoming and outgoing data rates synchronized.
A gapped signal by its nature carries significant amount of jitter, which usually is not tolerated by the downstream consumer circuitry. A jitter attenuating Phase Locked Loop (PLL) is typically used to create an output signal that has the same average frequency as the gapped signal with the jitter component attenuated. When the jitter component is interspersed in the gapped signal at a relatively high frequency, it can be effectively attenuated by the PLL. In addition, jitter with short durations can be removed more easily than those with long periods.
A frequency synthesizer may utilize a gapper and a jitter attenuating PLL to receive an input signal with frequency fi and generate an output signal with frequency fo, where D=1/R=fi/fo is usually a rational number. By virtual of the gapping operations, the average frequency of the gapped signal is lower than the corresponding input signal. Hence conventionally, frequency synthesizers that employ gappers are limited to applications that require D>1 (or R<1). This is disadvantageous in the applications where a higher frequency of output signal than the input signal is needed. For example, a clock signal transmitting from STS-192 to OTU2 needs to change from a frequency of 622.08 MHz to a frequency of 669.326 MHz.